What should I observe when pruning a lilac?

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After our lilac has flowered, I would like to prune it back by approx. 1-1.5 m, as it has become too large. The aim is that it should grow back again, but lower and bushier. How do I need to prune it, and when is the best time to do this?

The GARDENA gardening expert

Dear Mr. B., you have a choice of two options. 

The first and most obvious is to cut back the lilac to the correct height so that it branches out. To to do this, select as young a fork in the branch as possible at the correct point and height, cut the lilac back to this level and then use this point as a reference dimension to cut back the entire plant evenly all round. In this way, your lilac remains roughly in shape and above all does not receive the obligatory type of pruning in which the plant is mercilessly cut back along a line. When adjusting the height, you may achieve more natural proportions and therefore a more attractive appearance by reducing the width as well. 

Variation 2 would be a severe pruning right back into the mature wood. Lilac can cope with this. In this variation, prune back the plant radically to approximately one or one and a half metres in height or below, and allow it to grow again. You should support this new growth by supplying the plant with compound fertiliser. Use the new growth to rebuild the plant. Excess and incorrectly positioned young shoots should be removed, and the remaining shoots, if too long, should be shortened to approximately 5 nodes. The reconstruction of the plant should be conducted with a certain amount of imagination regarding how the plant should look.
The advantage of the second variation is that you can reconstruct the plant suitable for your garden situation, although you will initially lose out on some flowering periods. Variation 1 is the smarter method, but if you choose this, you will have to prune the plant sooner again than with variation 2. A lilac can make up for pruning by one to one and a half metres, as you have planned, within three to five years.

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